The present invention relates to an improved anti-corrosive material. Particularly, the present invention relates to a protective material for preventing microbiologically influenced corrosion in buried conduits. More particularly, the present invention relates to a multi-layered material used to encase buried conduits to prevent corrosion.
Buried conduits are ubiquitously used for carrying various materials, such as water, natural gas, oil, and sewage. A major problem with buried conduits comprised of metal or concrete with metal reinforcements is corrosion. The severity and rate of corrosion is dependent on the type of material comprising the conduit and the environment in which the conduit is buried. Insuring the longevity of buried conduits is an important part of the infrastructure in the United States and the world. Significant costs are involved in design, development, manufacture, and installation of water, gas, and sewage systems. Failure of these systems from conduit corrosion represents appreciable costs.
The control of corrosion of metals has been a quest of producers and consumers for the entire history of ferrous materials. Corrosion is typically understood to be the result of oxidation and/or galvanic processes. Historically, corrosion prevention has primarily addressed the reduction of oxygen component or control of galvanic action to prevent the occurrence of corrosion. Some typical corrosion prevention processes are painting or coating the metal to prevent oxygen from reaching the surface, volatile corrosive inhibitors that remove the available oxygen, and/or cathodic protection. Even after utilizing all the currently available mechanisms for the prevention of corrosion there are instances where unexplained corrosion occurs in buried metal pipes and conduits.
In the 1930""s, the mechanisms of Microbiologically Influenced Corrosion (xe2x80x9cMICxe2x80x9d) were proposed by Von Wolzen Kuhr to explain corrosion initiated or accelerated by microorganisms. Since that time, studies have shown the Von Wolzen Kuhr theory to be valid, and it has been established that a consortium of microorganisms contribute to many metal corrosion failures. These microorganisms, alone or more typically in combination, include as follows:
Sulphate-reducing bacteria including Desulfovibrio, Desulfobacter, and Desulformaculum. Sulphate-reducing bacteria are anaerobic and are the primary cause of Microbiologically Influenced Corrosion. Sulphate-reducing bacteria are associated with the reduction of sulphate under anaerobic conditions and an associated production of hydrogen sulfide, which creates an alkaline environment that can accelerate corrosion.
Iron-oxidizing bacteria including Gallionella, Sphaerotilus, Leptothrix, Clonothrix, and Crenothrix. Iron-oxiding bacteria are associated with the oxidization of various forms of iron and, in some cases, an associated production of ferric chloride and an acidic environment that can accelerate corrosion.
Sulfur-oxidizing bacteria including Thiobacillus, Thiodendron, Beggiatoa, and Sulfolobus. Sulfur-oxiding bacteria are aerobic bacteria that form sulphuric acid, which is corrosive to many metals, from the oxidation of sulphur or sulphur-bearing compounds.
Slime-forming bacteria including Pseudomonas, Escherichia, Flavobacterium, Aerobacter and Bacillus.
Susan Watkins Borenstein, Microbiologically Influence Corrosion Handbook ch. 2 (1994).
Hereto, no one has effectively addressed the elimination of the microbiologically influenced corrosion occurring in buried metal pipes and conduits. For example, ductile iron pipe (DIP) typically exhibits a low risk to severe corrosion compared to other metals; however, a rapid increase in the corrosion rate can be initiated by oxygenated water, tidal action, or specific soil types such as soils containing sulfides. Because of the high costs associated with removal and replacement of corroded conduits, the industry has expended substantial resources attempting to solve this problem.
Initially, conduits were covered with paint coatings, wraps, or other materials to separate the conduit surfaces from the environment. However, specialized coatings are either susceptible to deterioration by sulfate-reducing bacteria or are sophisticated to the point that they are no longer cost effective. Later, barrier films of polyethylene were used to protect DIP conduits. By insulating the exposed surfaces from soil, electrical currents, and oxygenated water, corrosion is usually prevented. However, due to improper installation, tears and punctures to the barrier film occurring during the installation and backfill process, free flow of water from tidal action, or soil or water becoming entrapped between the film and the conduit surface, actual corrosion still occurs in many cases. The industry has attempted to solve these problems by using more durable barrier films to encase the conduit surfaces, such as high density cross-laminated polyethylenes (HDCLPE). The superior impact strength, tear resistance, and tensile strength of HDCLPE has reduced some of the problems associated with the installation and backfill process; however, HDCLPE does not adequately address or control the problem of microbiologically influenced corrosion. Since there has not been an adequate alternative, present industry standards typically use either an 8 mil low density polyethylene (LDPE) film or a 4 mil HDCLPE film, a mil being equal to one thousandth of an inch (0.0254 millimeter), to wrap around the conduits for protection against corrosion.
Polyethylenes, as well as other plastic films, limit the free flow of water against the conduit surfaces, thereby reducing available oxygen. Any moisture that becomes trapped between the film and the conduit surface will eventually become deaerated. A problem arises where deaerated water levels are attained in the presence of the previously identified sulfate reducing bacteria. Many anaerobic bacteria, such as Desulfovibrio desulfuricans, thrive in certain fresh water, brackish water, sea water, sulfate soils, or warm soil conditions. These bacteria act as a catalyst to initiate or augment the rate of corrosion in an environment that is normally adverse to corrosion, and as previously noted are a primary cause of microbiologically influenced corrosion. Additionally, other types of bacteria are believed to play a part in corrosion propagation and it appears that bacteria are also responsible for degradation of the polyethylene film. A possible solution to this problem is to treat the materials used to encase the conduit with bactericides. However, most bactericides are topical and water soluble, thereby offering only initial protection that loses their effectiveness when used in buried systems exposed to wet conditions. Since conduits are buried for decades, this would not provide adequate long-term protection.
Another possible solution is to use certain volatile corrosion inhibitors (VCIs) which can be introduced either in contact or near the metal surface to eliminate or reduce the presence of corrosion. An example of a commonly used VCI is illustrated in U.S. Pat. No. 3,425,954. These VCIs can be used to prevent conditions from developing inside the film barrier which are favorable to corrosion. VCIs work at a micron level to passivate the surface of metal with a passive film, thus reducing the chemical reactivity of its surface. VCIs are normally used in kraft papers for short term protection of metal parts, as illustrated in U.S. Pat. No. 4,557,966; however, paper is not suitable to be buried. VCIs could be added to the polyethylene film, but the effectiveness will be shortened since the vapor tends to escape from the film, thus preventing extended protection. VCIs also lose their effectiveness when used in buried systems. An additional possible solution is the practice of isolating the system to prevent corrosion due to galvanic action; however this technique is ineffective against bacterial corrosion. However, currently available methods for controlling conduit corrosion do not address microbiologically influenced corrosion, or their application is technically complex and very expensive, or they are not suitable for buried conduits
From the foregoing it may be seen that hereto, no one has adequately addressed the elimination of the microbiologically influenced corrosion element for the long-term protection of buried conduits. A need exists for an improved anti-corrosive material for protecting conduits buried in conditions favorable to microbiologically influenced corrosion.
It is the object of the present invention to provide an improved anti-corrosive material which is superior to those presently used to protect buried conduits.
It is an additional object of the present invention to provide an improved anti-corrosive material that provides long term protection of buried conduit by preventing microbiologically influenced corrosion.
It is another object of the present invention to provide a material that contains an antimicrobial additive to prevent bacterial-induced or enhanced corrosion.
It is a further object of the present invention to provide a material that contains volatile corrosion inhibitors to prevent corrosion.
It is another object of the present invention to provide an anti-corrosive material comprising a conduit-contacting layer having an antimicrobial additive impregnated therein, incorporated into and dispersed throughout the conduit-contacting layer such that the antimicrobial additive can migrate within the conduit-contacting layer to contact the conduit surface and thereby prevent microbiologically influenced or enhanced corrosion therein.
It is another object of the present invention to provide a multi-layered, anti-corrosive material having a barrier layer adjacent the conduit-contacting layer that prevents the antimicrobial additive from penetrating the barrier layer and entering the surrounding environment.
It is still a further object of the present invention to provide a material which is co-extruded, calendered, or laminated into a multi-layered material comprising a conduit contacting layer having antimicrobial additives impregnated therein, incorporated into and dispersed throughout the conduit contacting layer such that the anti-corrosive agents can migrate within the conduit contacting layer to contact the conduit surface and prevent corrosion.
It is yet a further object of the present invention to provide a material which is co-extruded, calendered, or laminated into a multi-layered material further comprising a barrier layer adjacent the conduit contacting layer which prevents the anti-corrosive agents from penetrating the barrier layer and escaping into the surrounding environment.
These and other objects of the present invention are accomplished through the use of an improved anti-corrosive material used to protect buried conduits from corrosion. Incorporated into the improved anti-corrosive material are antimicrobial additives that effectively eliminate bacteria associated with microbiologically influenced corrosion. The additives can be blended with polyethylene raw materials in a manner that disperses the material throughout the product and be insoluble in aqueous conditions. Since the additive is designed as a contact agent, a requirement of manufacture is that proper dispersion and percent content be effected to generate the proper barrier against microbial growth. A test program was implemented to develop the proper polyethylene film manufacturing techniques that would yield proper protective characteristics. This product is a new cost effective anti-corrosive material used to protect buried conduits comprising ductile iron, steel, or concrete reinforced with steel by reducing or eliminating previously unexplained failures of buried conduits caused or accelerated by microbiologically influenced corrosion.
The anti-corrosive material is comprised of a conduit contacting layer of polyolefin having the anti-corrosive agents impregnated therein. The anti-corrosive agents, or antimicrobial additives, are incorporated into and dispersed throughout the conduit contacting layer such that the anti-corrosive agents can migrate within the conduit contacting layer to contact the conduit surface and prevent corrosion.
Another embodiment of the anti-corrosive material is a multi-layered co-extruded, calendered, or laminated polyolefin. The material has an outer layer, or environment contacting layer, preferably comprised of a low density polyethylene having characteristically strong tensile strength and elongation properties to provide conventional protection from soil, water, air, or other potentially damaging elements. The material has a center layer preferably comprised of a high density polyethylene having superior tensile strength to provide a high density barrier between the outer layer and an inner layer. The inner layer, or conduit contacting layer, is preferably comprised of a low density polyethylene impregnated with an antimicrobial additive such as a biocide or a volatile corrosion inhibitor (VCI), or both. The antimicrobial additive is incorporated into and dispersed throughout the conduit contacting layer for killing corrosion-inducing bacteria on the surface of the conduit. The antimicrobial agents are able to migrate through the low density polymer matrix but the rate of migration of the antimicrobial additive is considerably slowed through the high density polymer and the VCI is essentially unable to penetrate it. Thus, the antimicrobial agents are substantially prevented from escaping into the surrounding environment, but rather, are trapped within a xe2x80x9cprotection zonexe2x80x9d immediately adjacent the conduit surface to provide extended protection against corrosion.
The present invention is not limited to simply overcoming the deficiencies of the prior art but introduces enhanced technical advantages in corrosion protection that were not previously available for the long term protection of buried conduits while continuing to meet established industry standards.
These and other objects and advantages of the invention will become apparent from the following detailed description of the preferred embodiment of the invention.